Something I learned from an old wedding photographer.. Don't cut your prices if you're trying to make a living.. Of course photgraphers have a bit more overhead other than just their time and tools (camera or guitar depending)...
But if you're good, you will get what you demand.
I don't know how to relate pounds to US$, but keep in mind.. 100 may be a good deal on the wedding itself, but and forget about the practice, which you would need to attend, that will last several hours the day or night before. If they expect you to be at the reception as well, you might want to more than double that fee.. Last wedding I went to was an hour from the time we got there until we left and it was a pretty quick wedding with no attendants, candle lightings, etc.. The string trio was there for about an hour and a half.. The reception however lasted from around 6pm until well after midnight.. Of course that had a full band for that, that played about 3 hours total, but had to be there setup well before the guests and couldn't tear down until after the guests were gone... They threw one hell of a party..
Think about what your time is worth and charge by the hour.. If you think 100 is fair for the wedding (and maybe the practice) that'll take up 4 hours of your time.. Let 'em know that they get you and 4 hours of music, each additional hour at 15 - 25 or whatever you think is fair or whatever your market will handle...
Anyroads... Good luck with the endevor...
Oh yeah.. Schmoozing in a bar sound's like a good idea... I wouldn't do it at a wedding.. Keep cards handy in case you're asked, but don't try and promote yourself in anyway other than your playing there.. A wedding is not about you, it's about the couple getting married..
--
Rob